Often, a safety device is designed in such a way that it can be triggered by the entry of a human extremity or other objects into a predefined area. Unfortunately, those safety devices can be prone to generating false danger signals, that is, they might indicate an unsafe condition exists when, in fact, no such condition is present. In many cases, false alarm signals might be generated due to an inability of the safety device to distinguish between different objects within the area that the safety device monitors. The present invention is designed to distinguish between materials based on measurable characteristics of the monitored areas.